1. Field of the Invention
This invention relates to an electronic toll collection system based on radio communications for a tollgate, and particularly relates to a communication method suitable for the radio environment of a tollgate.
2. Description of Related Art
An electronic toll collection system (ETCS) is an electronic system that enables drivers at a tollgate on a turnpike such as an expressway to receive tickets or pay their tolls without stopping their cars.
FIGS. 2A and 2B show an outline of a tollgate which uses an ETCS. FIG. 2A is a perspective view and FIG. 2B is a top view. The tollgate 300 is demarcated from the neighboring gate zone by an island 311. A roadside communication antenna 21 is installed on the tollgate 300. As a car 31 with a vehicle-mounted communication antenna enters a designed communication area 350, necessary information for charging is exchanged between the roadside communication antenna 21 and the vehicle-mounted communication antenna 22 through radio signals and the car 31 can pass through the tollgate without a stop.
If a car which has no vehicle-mounted communication antenna 22 enters the tollgate area, a car sensor 321 detects its entry and, if no communication with the roadside communication antenna 21 occurs, the car is considered as having no vehicle-mounted communication antenna 22 and the driver must receive a ticket from toll collection personnel in the tollgate booth 301 or pay the toll to him or her.
In this system, if a car with a vehicle-mounted communication antenna enters the tollgate area and no communication is established, the car must stop temporarily. Conversely, if a car without a vehicle-mounted communication antenna enters the zone and a communication between the roadside antenna and the vehicle-mounted communication antenna of another car is established, the former car can pass through the tollgate without paying the toll. Therefore, in order to ensure that tolls are collected without fail, it is proposed that the designed communication area should be set to allow only one car to enter it and there should be a means to enable communication only with the vehicle-mounted communication antenna which is present in it, or disable communication with any vehicle-mounted communication antenna outside it.
For example, the toll collection system disclosed in J-P-A-No.40433/1998 uses a roadside communication antenna which irradiates electromagnetic wave beams with high directivity; and J-P-A-No. 214359/1998 discloses a system in which a car type detector is installed at the front of the tollgate and the directional pattern of electromagnetic wave beams of the roadside communication antenna is varied depending on the car type for the purpose of suppressing communication area variation caused by variation in the position (vertical) of a vehicle-mounted communication antenna.
Another type of proposal is that the roadside antenna should be selected depending on the car""s position or height, or depending on the car""s height and speed (J-P-A-No.315283, 5/1992 and J-P-A-No.239954/1995).
In the conventional toll collection systems, in order to ensure that each communication is established with only one car at a tollgate, attention is paid only to the directional range of direct electromagnetic wave beams but the influence of reflected waves in the communication area is not taken into consideration.
As shown in FIG. 2A, actually there are structures such as a sound-proof wall 201 and a roof 221 in the tollgate area. As the roadside communication antenna 21 irradiates an electromagnetic wave beam, not only direct radio wave 210 which comes directly from the antenna 21 reaches the vehicle-mounted communication antenna 22 in the car 31, but also reflected wave 211 from the roof 221 and reflected wave 212 from the sound-proof wall 201 may be generated. Since the vehicle-mounted communication antenna 22 receives a radio signal as the direct wave combined with the reflected waves, the direct wave 210 may be offset by the reflected waves 211 and 212 even within the directional range of the electromagnetic wave beam and thus there may be a zone where the radio signal cannot be received. Such a zone momentarily changes depending on the position, speed and other factors of the car 31 and the next car 32.
Besides, even when the vehicle-mounted communication antenna 23 in the next car 32 is outside the directional range of the roadside communication antenna 21, the next car 32 may receive reflected waves from the structures in the tollgate area and/or the car 31 ahead, generating a path of reflected waves 213, which means establishment of communication with a car outside the designed communication area. In this case, because the roadside communication equipment cannot distinguish between the car 31 and the next car 32, if the communication with the latter is established before establishment of communication with the former, the equipment would mistake the communicating car 32 for the car 31 and, therefore, may allow the car 31 to pass through the gate without communication with it or without charging it. The frequency of such mistakes will be higher if there are more cars without vehicle-mounted communication antennas which enter the designed communication area.
As mentioned above, the communication area in conventional ETCSs is designed on the premise of transmission and reception of direct radio waves or at most once reflected wave from the ground, so reflected waves which vary depending on the entering car or other factors may make communication in the designed communication area impossible or make communication outside the area possible. This leads to the problem of low stability and low reliability in radio wave transmission and reception for toll collection.
Also, a special tollgate structure and a wide gate are required to decrease reflected waves. In addition, a specific communication means for each tollgate may be needed because different tollgates have different electromagnetic field environments. This results in a higher construction or operating cost.
The present invention has been made in view of the above circumstances and provides a toll collection system which electronically collects tolls with accuracy using special software to ensure stable communication with a car in the designed communication area while the radio environment in the tollgate area varies depending on the car position, and also provides its communication method.
The present invention concerns a toll collection system which charges cars which pass through a tollgate without a stop, by means of radio communication. The system is composed of the following devices installed at the tollgate: a roadside antenna whose directional pattern is variable; roadside communication equipment including a communication controller, which communicates with cars and the host system; and a car sensor which detects the position and profile of the car going to pass through the gate. In the system, taking it into consideration that the radio communication environment as mentioned above is influenced by reflected waves which vary depending on the car""s position and profile, the environment for communication between the roadside antenna and the vehicle-mounted antenna in the designed communication area is maintained suitable for automatic toll collection.
For this purpose, the system is characterized in that plural paths of direct and reflected waves which connect the roadside antenna with the antenna in the car present in a given area (car detection area) including the communication area are found and one path which maximizes the sensitivity of communication with the car in the communication area is selected from among them, then the directional pattern of the roadside antenna is so adjusted as to suit the direction of radiation (radiation direction 1) for the selected radio wave path. Also the directional pattern is controlled so that radio communication is impossible with respect to a wave path radiation direction (radiation direction 2) other than radiation direction 1, namely so that the radiant intensity for radiation direction 1 is maximized (or more than intensity 1) and that for radiation direction 2 is null.
If the presence of more than one car in the car detection area is detected, plural paths of direct and indirect radio waves for each car are calculated and all paths of radio waves from/to any car outside the car detection area are treated as having the radiation direction 2.
Regarding the radio wave paths, the system calculates paths of direct waves and all reflected waves available for communication that connect the roadside antenna with the vehicle-mounted antenna whose position depends on the position of the detected car, according to the known reflecting surface data based on the tollgate structure and the reflecting surface data which varies depending on the car profile. A radio wave path for direct waves can be geometrically calculated from data on both the antennas and the reflecting surface data, while ones for reflected waves can be calculated by a numerical method based on radio wave data. Alternatively, it is also possible to calculate paths for different car profiles in advance on the assumption that a car is in the position of one of plural path points which are preset in the car detection area.
The process for calculating the radio wave paths begins when a car enters the communication area, and is repeated for every cycle of detection of the car or every path point. As radio communication with the car in the communication area is over, the process may be once ended.
If radio communication with the car in the communication area is not established, the system considers the car as not having a vehicle-mounted antenna adequately. This means that if communication is impossible even though the communication environment is good, the system considers that the car has no vehicle-mounted antenna or the vehicle-mounted antenna is not ON, judges the car unsuitable for automatic toll collection, and treats it as such, for example, by giving a warning.
As described above, according to the present invention, toll collection can be performed with accuracy because the influence of reflected waves from a tollgate structure or an approaching car on radio waves between the roadside antenna and vehicle-mounted antenna is avoided and a desirable radio wave path for communication with a car in the designed communication area is thus obtained with stability. Since the influence of reflected waves is avoided by adjusting the antenna directivity, restrictions on the tollgate structure can be relaxed and the tollgate construction or operating cost can be reduced.